1. Field of the Invention
This invention relates to underwater cryogenic pipeline systems. In particular, the invention relates to underwater liquefied natural gas pipeline systems for use in ice infested waters to transfer liquefied natural gas between an onshore production or storage facility and an offshore vessel.
2. Description of Related Art
Liquefied natural gas (LNG) is a cryogenic liquid. At atmospheric pressure its temperature is about -162.degree. C. (-260.degree. F.). The principal design considerations for LNG pipelines are discussed below. First, the pipeline material must have sufficient ductility and toughness to be usable at cryogenic temperatures. Only aluminum, high nickel content steels and austenitic stainless steels are suitable.
The second principal design consideration is contraction of the pipeline. An LNG pipeline constructed at ambient temperatures and filled with LNG will contract. For example, a pipeline made of 304 stainless steel will contract about 13 feet per mile when cooled from 21.degree. C. (70.degree. F.) to -162.degree. C. (-260.degree. F.). If the pipeline is restrained at both ends and cooled as above, the stress resulting from contraction will exceed the allowable stress in the pipeline material. The same is true for aluminum and high nickel (9%) steels. It is therefore necessary to have some type of expansion joint for compensating for expansion and contraction. Pipe loops, shown in U.S. Pat. No. 3,379,027 are one well know type of expansion joint. Others are discussed below.
U.S. Pat. Nos. 3,388,724 to R. W. Mowell, et al. and 3,885,595 to Gibson, et al illustrate bellows type expansion joints. Contraction is taken up by a longitudinal bellows or corrugation in the inner pipe. The bellows is constructed out of a material that is relatively thinner than the material of the LNG pipeline so the bellows is free to expand and contract axially with respect to the LNG pipeline. A variation on the bellows expansion joint is snake pipe, such as that manufactured by Tokyo Rasenkan Seisakusko, Ltd. Snake pipe consists of a bellows formed out of relatively thick gauge material. The thicker material should yield increased reliability over thin-walled bellows. However, the thick bellows acts as a powerful spring and will induce higher axial loadings than bellows when the snake pipe is compressed axially during construction of the system.
One material, 36 percent nickel steel, also known as INVAR, is a candidate for use in LNG pipelines due to its low coefficient of thermal expansion. Because INVAR has such a low coefficient of thermal expansion, an INVAR pipe can be restrained at both ends and used for LNG service without exceeding allowable stress limits for the INVAR, thus the use of bellows or expansion loops can be avoided. However, experience with large diameter INVAR pipe is limited and present design codes do not include INVAR as cryogenic pipeline construction material.
The third principal design consideration associated with LNG pipelines is heat transfer. LNG pipelines must be well insulated to reduce heat leak and excessive LNG vaporization.
The fourth design consideration is anchoring. Whenever there is a change in the direction of the pipeline, for example when a horizontal pipe is attached to a riser, the reaction forces resulting from the hydraulic forces must be taken by anchors. Furthermore, the weight of the pipe together with the product needs to be anchored independently from the casing pipe and insulation. Anchoring is usually accomplished by attaching heavy steel or concrete anchors buried in the ground to the pipeline at regular intervals.
The above problems have been overcome with at least some success in aboveground LNG pipelines. An example is the Brunei LNG project pipeline, described in "Brunei LNG Plant and Loading Facilities", F. Frieseman, Session 4, Paper 3, LNG-2 The Second International Conference and Exhibition, 1970, Paris, France; and in "An LNG Ship Loading Pipeline", R. E. Crowl, Applications of Cryogenic Technology, Volume 4, pages 127-144. The Brunei pipeline consists of two 304 stainless steel 18" LNG lines extending about 21/2 miles offshore to a tanker loading facility. The pipelines are supported above the water by a trestle structure. Bellows type expansion joints are located every 36.5 meters (120 ft.) on the pipeline and the pipeline is attached to the trestle adjacent the expansion joints.
To ship liquefied natural gas (LNG) by sea, a way to transfer LNG between shore-based storage tanks and sea-going tankers is required. Where deepwater near-shore port facilities are available, it may be feasible to construct an aboveground pipeline from the LNG production or storage facility to the dock side. In some cases, such as in Brunei, an above-water jetty out to the offshore LNG transfer facilities can be built. A conventional LNG pipeline is then laid on the jetty. Obviously, this approach will not be feasible where LNG loading facilities must be located far offshore in deep water or in areas, such as the Arctic, where ice movement can impose large forces on above water structures. Building a jetty strong enough to withstand ice forces would probably make this approach too costly. Thus, an underwater LNG pipeline will be preferred when offshore loading facilities are far offshore in deep water or ice infested areas, such as the Arctic.
Three principal systems for underwater transport of LNG are known. They are the tunnel system, the roller supported retrievable line and the INVAR pipeline. Several published articles have described the underwater LNG pipeline tunnel constructed at Cove Point, Md. during the late 1970's. The Cove Point tunnel consists of a large, approximately one mile long, concrete tunnel laid on the sea bed. The tunnel contains two passageways for LNG lines and a smaller maintenance passageway. The Cove Point tunnel contains four lines in two pairs. Each pair is contained in a separate passageway in the tunnel. The maintenance passageway is sealed-off from the LNG passageways by pressure tight doors. Water infiltration into the insulation on the lines is not a problem because the LNG lines are always dry. LNG leaks are readily detected and repairs through the access passageway are relatively easy. The primary disadvantage of a tunnel system is that it is extremely expensive to construct.
The second prior art system is the retrievable line. U.S. Pat. No. 3,379,027 describes this system in detail. An inner LNG line is supported on rollers in a large casing so the LNG line can be disconnected and rolled out of the casing. The retrievable line could be simpler and less expensive than the tunnel. Repairs would be made to the lines by retrieving them. However, there is no history behind this system which has never been constructed.
U.S. Pat. No. 4,133,181 to Kotcharian proposes a two-section cryogenic pipeline from the shore to offshore loading facilities. The first pipeline section is supported on pillars above the water level. The pipelines are connected together with cross braces to form a stiff frame that can span between the pillars above the water. The second segment of the system is connected to the first segment and consists of prefabricated underwater pipeline segments and a number of spaced-apart pillars extending above the surface of the water. The underwater pipeline segments are routed up and over the pillars so the connections between the segments can be made above the surface of the water. There are several disadvantages with Kotcharian's system, as follows. Kotcharian states that expansion joints are utilized every 200 meters. Expansion joints would be needed every approximately 36 meters, based on the capability of expansion joints currently available. There is no apparent advantage to submerging the pipeline, since the system needs above water pillars to connect and anchor the underwater segments. Kotcharian's system will probably not work in deep water where fabricating the necessary pillars would make the system too costly. In addition, the pillars and the pipelines are susceptible to damage by moving ice in the Arctic.